Field of the Invention
The present invention relates generally to a photolithographic method using a photo-resist, and more specifically, to a photolithographic method using a chemically amplified photo-resist.
Description of the Related Art
With the rapid development of the microelectronic industry, critical dimensions of a semiconductor device continue to shrink. The shrinkage of the critical dimensions of a device depends on exposure tools. The shrinkage is also closely related to the selection of a photo-resist. Thus, the selection and application of a photo-resist corresponding to photolithography also become an important research content for photolithographic processes.
The advancement of photolithography promotes the steady improvement in performances of a photo-resist. The photo-resist using a chemical amplification action has many advantages such as high sensitivity and strong ability to withstand dry corrosion, which facilitate the subsequent processing process of the semiconductor. A chemically amplified photo-resist has thus a broader application prospect in the semiconductor manufacturing field and gradually gains attention in the photolithographic field. It is believed that a chemically amplified photo-resist with its steady processing properties will play an important role in the semiconductor industry.
A chemically amplified photo-resist generally comprises three components: a matrix resin, an organic solvent, and a photoacid generator (PAG) for producing a chemical amplification action. After the chemically amplified photo-resist has been exposed to or illuminated with light, the PAG absorbs energy and undergoes a photolysis. Thus, free acid is generated, which results in an acid catalytic reaction such that exposed portions of the matrix resin will undergo a removal reaction of protecting groups or a cross-linking reaction between resin and cross linker, forming positive or negative latent images which are then subjected to development in a certain solvent to form exposure images. In addition, some chemically amplified photo-resist may employ a photobase generator (PBG) instead of a photoacid generator. An alkaline catalytic reaction takes place with the help of photobase, which likewise results in that the matrix resin undergoes a removal reaction of protecting groups or a cross-linking reaction between resin and cross linker, forming a positive or negative latent image.
However, the contrast of the latent image will be degraded due to following factors: One factor is photoacid diffusion. The photoacid generated by illumination with a light in a first wavelength band will gradually diffuses from a position of high mass concentration to a position of low mass concentration through free movement of molecules. In this way, the mass concentration distribution of the photoacid will depart from the optical image, thereby degrading the contrast of the latent image of the photoacid.
The other factor is photo diffraction. Theoretically, an optical image formed by means of a mask should be a simple binary image, that is, in the optical image, the light intensity of a portion of the image where the light is shielded by the mask is zero, while the light intensity of the other part of the image where the light transmits through the mask is a constant. However, with the continuous shrinkage of the critical dimensions for semiconductor processes, light diffraction effect becomes more and more evident, such that the portion of the optical image that should have a light intensity of zero also has certain light intensity. As a result, the contrast of the latent image of the photoacid is further degraded.
In the prior art, a diffusion length or depth is restricted to enhance the contrast of a latent image. However, the restriction method is disadvantageous since it will make the removal reaction or the cross-linking reaction less efficient. Besides, the conventional method also fails to overcome the degradation of the contrast of a latent image caused by the diffraction effect.